RHIC Experimental Program-Background in preparation for meeting

Wit Busza

MIT

Hard Probes 2004

Lisbon, Portugal

Many thanks to Michael Miller for help in preparation of this talk, to Conor Henderson for making some of the transparencies, and to the four RHIC collaborations for most of the material presented in the talk.

The Relativistic Heavy Ion Collider at BNL

RHIC First Commissioned, June 2000

2000-2004 RHIC PERFORMANCE

Phenix 1370 b-1

Star 1270 b-1

Phobos 560 b-1 Brahms 540 b-1

56GeV/u

RHIC Run Colliding System √sNN

1

Au+Au

Au+Au

55.87GeV

130.4GeV

2

Au+Au

Au+Au

Au+Au

p+p

130.4GeV

200.0GeV

19.59GeV

200.0GeV

3

d+Au

p+p

200.7GeV

200.0GeV

4

Au+Au

Au+Au

p+p

200.0GeV

62.40GeV

200.0GeV

THE DETECTORS

Coils Magnet

E-M Calorimeter

Time of    Flight

Projection           Chamber

STAR Solenoidal field, large- tracking

TPC’s, Si-vertex trackingRICH, TOF, large EM Cal

~420 participants

PHENIXAxial field, high resolution & rates

2 central arms, 2 forward muon arms TEC, RICH, EM Cal, Si, TOF, -ID

~450 participants

The Two Large Detectors

BRAHMS2 “conventional” spectrometers

Multiplicity detector with large phase space coverage

Magnets, TPCs, TOF, RICH~40 participants

PHOBOS“Table-top” 2-arm spectrometer

full phase space multiplicity measurementMagnet, Si pad detectors, TOF+dE/dx

~70 participants

Spectrometer

Paddle Trigger Counter

137000 Silicon Pad ChannelsT0 counter

NIM A 499, 603-623 (2003)

Time of Flight

Calorimeters

Octagon

The Two Small Detectors

High Multiplicity Au+Au Collision at sNN=130 GeV

colors ~ momentum: low - - - high

STARSTAR

PHOBOS PID & ACCEPTANCE

Measured Quantities

Single Particles

Tracking p

time of flight, massdx

dE } P

Event Characteristics

Forward Neutral Energy

d 2nch

dd

dET

dd }

nchtotal

ETtotal}

“spectators”

Centrality or impact parameter, b

[“Npart”, “Ncoll”]

“Spectators”

Zero-degreeCalorimeter

specpart 2 NAN

Paddle Counter

“Participants”

Experimental Control of Centrality or Impact Parameter

“Spectators”

Bi-product: Study of Mechanism of Particle Production

See, for example, W. Busza arXiv: nucl-ex/0410035

Aim of Research:

Too good to be true!

From Harris and MuellerAnn. Rev. Nucl. Sci. 1996

But RHIC program is an incredible success

RIKEN BNL Workshop May 14-15, 2004

Submitted to NPA 2004

New Discoveries at RHICA RIKEN BNL Research Center Workshop, May 14-15, 2004

Proceedings, Volume 62, BNL-72391-2004

BRAHMS, Phenix, Phobos & STAR “White Papers”:

“Perspectives on Discoveries at RHIC”

To be submitted to NPA November, 2004BRAHMS arXiv: nucl-ex/0410020

Phenix arXiv: nucl-ex/0410003

Phobos arXiv: nucl-ex/0410022

Where Are We Now?

In Au + Au Collisions at RHIC

1. In 1 fm/c energy density≥3GeV/fm3

2. Description of the created system in terms of simple hadronic degrees of freedom is inappropriate3. Constituents of this novel system are found to interact very strongly

In addition large body of high quality data has been collected on a broad range of topics. Much of it is not well understood. Phenomenology is often simpler than the interpretations.

PHOBOS

PHOBOS

Global Properties

PHOBOS

Data smooth as a function of energy

Pseudorapidity plotted in rest frame of one of the nuclei

TOTAL CHARGED MULTIPLICITY

Elliptic Flow

PHOBOS nucl-ex/0406021 Au+Au 0-40%

Elli

pti

c F

low

Pseudorapidity plotted in rest frame of one of the nuclei

Data smooth as a function of centrality or impact parameter

Total number of particles PHOBOS

Mid rapidity density

NA49 (from Agnes Richard)

NA50 Experiment

SOME EXCEPTIONS:

Although in the RHIC Energy Range (20-200 GeV) there are no obvious discontinuities, with very

reasonable assumptions we can conclude

Time of equilibration is short

Energy Density is very high,

cfm133 fmGeV

Note: Cold Nuclear Matter Density ~

Energy Density Inside Hadrons ~

150 MeV fm3

500 MeV fm3

Not only is the energy density very high, the matter is strongly interacting at early times

Evidence:

Strong Flow Signal

Analogy: Elliptic Flow of Ultracold Li6 Atoms

K.M.O’Hara et al, Science, 298 (2179) 2002

T~ 50 10-9 K

Elliptic Flow a very real phenomenon!Elliptic Flow a very real phenomenon!

x

y

x

y

Elliptic Flow at RHIC

Geometry: asymmetric initial state

Fourier analysis 1+2v2cos2(lab-plane)

Asymmetry + interactions creates final state azimuthal correlations:

elliptic flowlab-plane

PHOBOS

Bulk PQCD Hydro qqq Coalescence

Large v2 - strongly interacting matter at early time

pQCD Jets

From Gyulassy DNP 2004

To answer these questions one needs localized penetrating probes

Such probes do exist main topic of this meeting

What is the novel medium?What is the origin of its strong interactions?

parton (quark or gluon)

parton (quark or gluon)

HARD PROBES

q+g q+g

For parton-parton scattering with high Pt

The basic interaction is understood pQCD

(Nobel Prize 2004)

pQCD

If parton beams of known momentum where available and scattered partons could be directly detected, life would be

beautiful!

q+g jet+ X

jet

hadron

p

pz

ionFragmentat

:Rcone=0.7 rad

The best we can hope to do is to look at jets or jet fragments. This is reasonably well understood (combination of calculation and phenomenology, eg e+e- annihilation)

pQCD

p+p π0+ X

“leading” π0

Xpp 0

More phenomenology needed but under control:

pQCD

PDF

PDF

Au+Au π0+ X“leading” π0

More complications:

Multiple interactions and radiation before high Pt scattering (e.g.Cronin effect),

Shadowing

Saturation (e.g. Color Glass Condensate)

Appropriate normalization?

Effect of medium on fragmentation?

pA &dA helps to sort out initial state and final state effects

Correct Normalization, i.e. What is the Number of Relevant Collisions for Colliding Nuclei?

Numbers obtained from Glauber model:Straight trajectoriesConstant cross-sectionNuclear density profile

Issues:Appropriate cross-sectionShadowingSaturationAll of above are Pt dependent

Npart = 4 + 2 = 6Ncoll = 4 2 = 8

Nuclear Modification Factors

Some vocabulary:

• Peripheral Au+Au = superposition of p+p reactions.

• Central Au+Au reveals a significant suppression !

EXAMPLES OF DATA

CentralPeripheral

“Jet quenching” or Suppression of High PT Particles

Phenix Phenix

STAR

PHENIX

PHOBOS

BRAHMS

AuAu and dAu comparison (Ncoll normalization):

BRAHMS

Phys Rev Lett 91, 072302/3/5 (2003)

STAR: Phys.Rev.Lett.91:072304, 2003

Back-to-back jets:

Out-plane

In-plane

K. Filimonov: DNP 10.31.03; nucl-ex/0410009

Correlation of suppression with reaction plane:

• Different particles may behave differently. Looking at the behavior of all charged particles together may be misleading.

• Theorists may be pushing their luck using a high Pt approximation to a regime where it is clearly not applicable.

• Most data are for single particles and yet one speaks of them as if they are scattered partons or jets

• Inappropriate or unknown normalization can enhance or suppress an effect

• Important to ask if a “high Pt effect” is not there at low Pt

•Is there evidence that jet quenching goes approximately like density times the square of path length?

THERE ARE ISSUES ONE SHOULD BE AWARE OF WHILE LISTENING TO THE TALKS

Suppression depends on produced particle type:

• Different particles may behave differently. Looking at the behavior of all charged particles together may be misleading

• Theorists may be pushing their luck using a high Pt approximation to a regime where it is clearly not applicable

• Most data are for single particles and yet one speaks of them as if they are scattered partons or jets

• Inappropriate or unknown normalization can enhance or suppress an effect

• Important to ask if a “high Pt effect” is not there at low Pt

•Is there evidence that jet quenching goes approximately like density times the square of path length?

THERE ARE ISSUES ONE SHOULD BE AWARE OF WHILE LISTENING TO THE TALKS

• Different particles may behave differently. Looking at the behavior of all charged particles together may be misleading

• Theorists may be pushing their luck using a high Pt approximation to a regime where it is clearly not applicable

• Most data are for single particles and yet one speaks of them as if they are scattered partons or jets

• Inappropriate or unknown normalization can enhance or suppress an effect

• Important to ask if a “high Pt effect” is not there at low Pt

•Is there evidence that jet quenching goes approximately like density times the square of path length?

THERE ARE ISSUES ONE SHOULD BE AWARE OF WHILE LISTENING TO THE TALKS

• Different particles may behave differently. Looking at the behavior of all charged particles together may be misleading

• Theorists may be pushing their luck using a high Pt approximation to a regime where it is clearly not applicable

• Most data are for single particles and yet one speaks of them as if they are scattered partons or jets

• Inappropriate or unknown normalization can enhance or suppress an effect

• Important to ask if a “high Pt effect” is not there at low Pt

•Is there evidence that jet quenching goes approximately like density times the square of path length?

THERE ARE ISSUES ONE SHOULD BE AWARE OF WHILE LISTENING TO THE TALKS

0-10% Central 200 GeV AuAu

PHENIX Preliminary

1 + ( pQCD x Ncoll) / phenix

backgrd Vogelsang NLO

AuAu→Direct photons consistent with Ncoll scaling

A inelastic cross-section consistent with Ncoll scaling

Trends seen in “suppression” or “nuclear modification” depend on normalization

62.4 GeV200 GeV

PHOBOSAu+AuCharged particles

Most Central

Npart scaling

Ncoll scaling

Most peripheral

Factorization of Energy/Centrality Dependence

62.4 GeV200 GeV

Mid rapidity density PHOBOS

• Different particles may behave differently. Looking at the behavior of all charged particles together may be misleading

• Theorists may be pushing their luck using a high Pt approximation to a regime where it is clearly not applicable

• Most data are for single particles and yet one speaks of them as if they are scattered partons or jets

• Inappropriate or unknown normalization can enhance or suppress an effect

• Important to ask if a “high Pt effect” is not there at low Pt

•Is there evidence that jet quenching goes approximately like density times the square of path length?

THERE ARE ISSUES ONE SHOULD BE AWARE OF WHILE LISTENING TO THE TALKS

A of pA hX

Barton et al Skupic et al -2 -1 0 y

From E451:Barton et al Phys Rev 27 (1983) 2580

Particle ratios in forward production of particles are independent of A

“Quenching” is seen in production of all particles in the very forward region of rapidity ( 2 units)

• Different particles may behave differently. Looking at the behavior of all charged particles together may be misleading

• Theorists may be pushing their luck using a high Pt approximation to a regime where it is clearly not applicable

• Most data are for single particles and yet one speaks of them as if they are scattered partons or jets

• Inappropriate or unknown normalization can enhance or suppress an effect

• Important to ask if a “high Pt effect” is not there at low Pt

•Is there evidence that jet quenching goes approximately like density times the square of path length?

THERE ARE ISSUES ONE SHOULD BE AWARE OF WHILE LISTENING TO THE TALKS

LISTEN WITH SOME SKEPTICISM

ENJOY THE MEETING

IT IS AN EXCITING ERA FOR THE FIELD

Last Words